From 449fa2b3b567dd17b23ac8de00334fc14c31ca96 Mon Sep 17 00:00:00 2001
From: Robert Grizzell <robert@grizzell.me>
Date: Sun, 30 Aug 2026 20:57:59 -0500
Subject: [PATCH] SimSerial: line-buffer stdout so log lines flush promptly
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SimSerial routes Serial output to stdout via putchar(). When stdout is not
a TTY (a pipe, a file, or under a service manager), C stdio defaults to full
buffering, so log lines don't appear until a ~4 KB buffer fills — forcing
callers to wrap the process in `stdbuf -oL`.

Set stdout to line-buffered once, at the first SimSerial construction (the
global `Serial` is built at static-init, before any output), so each
Serial.print line is flushed on its newline and reaches the terminal /
journald promptly. Guarded to run once: setvbuf() is undefined after I/O has
begun, and extra SimSerial instances may be constructed later (e.g. tests).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KzBaSxeXYbJBJ7EYyfR3Lb
---
 cores/ardulinux/linux/LinuxSerial.cpp | 19 +++++++++++++++++++
 cores/ardulinux/linux/LinuxSerial.h   |  6 ++++++
 2 files changed, 25 insertions(+)

diff --git a/cores/ardulinux/linux/LinuxSerial.cpp b/cores/ardulinux/linux/LinuxSerial.cpp
index 8cc349d..beed04b 100644
--- a/cores/ardulinux/linux/LinuxSerial.cpp
+++ b/cores/ardulinux/linux/LinuxSerial.cpp
@@ -262,6 +262,25 @@ namespace arduino {
 
     // --- SimSerial: stdout-backed simulated serial (used as the console) ---
 
+    /**
+     * Line-buffer stdout the first time a SimSerial is constructed.
+     *
+     * The global `Serial` instance is constructed at static-init, before any
+     * output, so this runs before the first write.  With line buffering, each
+     * log line is flushed on its terminating newline, so `Serial.print(...)`
+     * output reaches the terminal / journald promptly without wrapping the
+     * process in `stdbuf -oL`.  Guarded to run only once: setvbuf() is undefined
+     * once I/O has occurred, and additional SimSerial instances may be created
+     * (e.g. in tests) after output has already started.
+     */
+    SimSerial::SimSerial() {
+        static bool configured = [] {
+            setvbuf(stdout, nullptr, _IOLBF, 0);
+            return true;
+        }();
+        (void)configured;
+    }
+
     /** No-op: simulated port requires no hardware setup. */
     void SimSerial::begin(unsigned long baudrate, uint16_t config) {}
 
diff --git a/cores/ardulinux/linux/LinuxSerial.h b/cores/ardulinux/linux/LinuxSerial.h
index 5dbb12a..1f435a8 100644
--- a/cores/ardulinux/linux/LinuxSerial.h
+++ b/cores/ardulinux/linux/LinuxSerial.h
@@ -114,6 +114,12 @@ namespace arduino {
  */
     class SimSerial : public HardwareSerial {
     public:
+        /**
+         * Line-buffer stdout (once, at static init) so log lines reach the
+         * terminal / journald promptly, without needing an external
+         * `stdbuf -oL` wrapper around the process.
+         */
+        SimSerial();
         /** No-op; the simulated port is always "open". */
         virtual void begin(unsigned long baudrate) { begin(baudrate, SERIAL_8N1); }
         /** No-op. */